A known hydraulic drive system for an actuator in a working machine including a track device of crawler type, for example, a hydraulic excavator controls a delivery flow rate of a hydraulic pump (main pump) such that a delivery pressure of the hydraulic pump is higher by a target differential pressure than a maximum load pressure of a plurality of actuators. Such a hydraulic system is called a load sensing system. The load sensing system uses a pressure compensating valve that maintains a differential pressure across each of a plurality of flow control valves at a predetermined value. The load sensing system thereby ensures that hydraulic fluid can be supplied at a ratio corresponding to an opening area of each flow control valve during a combined operation that simultaneously drives multiple actuators, regardless of the magnitude of the load pressure of each actuator.
Patent document 1, for example, discloses one type of such a load sensing system. The load sensing system disclosed in patent document 1 includes a differential pressure reducing valve that outputs a differential pressure (hereinafter referred to as a differential pressure PLS) between a delivery pressure of a hydraulic pump and a maximum load pressure of a plurality of actuators as an absolute pressure. The output pressure of the differential pressure reducing valve is then introduced to a plurality of pressure compensating valves. A target compensating differential pressure for each of the pressure compensating valves is then set using the differential pressure PLS and control is performed such that the differential pressure across the flow control valve is maintained at the differential pressure PLS. This permits the following. Specifically, if a saturation condition develops in which the hydraulic pump delivers a short supply of the delivery flow rate during the combined operation that simultaneously drives multiple actuators as described earlier, the differential pressure PLS decreases according to the degree of saturation, so that the target compensating differential pressure of the pressure compensating valve, specifically, the differential pressure across the flow control valve becomes small. The delivery flow rate of the hydraulic pump can thereby be redistributed according to the ratio of flow rate required by each actuator.
In the hydraulic drive systems for working machines including track devices of crawler type, a hydraulic system called an open circuit system that includes an open center type directional control valve (flow control valve) is widely used. Such an open circuit system is typically arranged such that hydraulic fluid is supplied independently from two hydraulic pumps to right and left track motors to thereby enable traveling, as disclosed in patent document 2. In the hydraulic drive system disclosed in patent document 2, two hydraulic fluid supply lines that supply hydraulic fluid to two directional control valves for tracks from two hydraulic pumps are connected via a skew correction circuit. When right and left track control levers are operated all the way in a direction for either a forward or reverse travel, a valve device included in the skew correction circuit is placed from a closed position in a throttled open position and, at other timing, the valve device is retained in the closed position. This prevents operability at timings other than straight line traveling operation from being aggravated and allows skew traveling to be corrected for straight line traveling during the straight line traveling operation.